The present invention relates to ceramic honeycomb filters of wall-flow design, and more particularly to materials and methods for plugging or manifolding such filters as required to provide for the effective wall flow filtration of particulate-laden gases flowing therethrough.
Honeycomb structures formed from ceramic materials have a number of important uses including use as catalyst supports for controlling emissions from combustion engines and stationary pollutions sources such as power plants. They are also used as porous particulate filter bodies for the filtration of particulate-laden exhaust gases emitted by combustion engines such as diesel engines. In the latter case, the bodies are modified through the sealing or plugging of the ends of selected cells of the honeycombs to provide a manifolded “wall-flow” filter structure. The terms “sealed” and “sealing” as used herein refer to both porous and non porous means of closing selected open transverse cross-sectional areas of cells.
In current practice, the fabrication of such filters involves plugging or otherwise sealing one end of each of the channels or cells traversing a porous ceramic honeycomb body, a first subset of cells being sealed at a first or inlet end face of the honeycomb and the remaining cells being sealed at a second or opposing outlet end face thereof. A particulate-contaminated fluid such as an exhaust gas is supplied under pressure to the inlet face and enters the body via those cells which have an open end at the inlet face (i.e., “inlet” cells). Because these cells are sealed at the opposite end face (“outlet” face) of the body, the contaminated fluid is forced through the thin, porous walls into adjoining cells which are sealed at the inlet face and open at the outlet face (i.e., “outlet” cells). The solid particulate contaminant in the fluid which is too large to pass through the porous openings in the walls is left behind and a cleansed fluid exits the filter body through the outlet cells for use.
Most common in the case of honeycombs with square channel cross-sections is a masking design producing a checkerboard pattern of plugs in each end of the honeycombs, such that each inlet cell is surrounded on four sides by outlet cells, and vice versa. Examples of such filter bodies and other wall-flow filter designs that have been developed for these uses, are disclosed, for example, in European Patent No. 0 043 694.
For the mass production of plugged wall flow filters it is highly desirable to be able to seal selected cell ends as rapidly and as inexpensively as possible. The hand-plugging if individual cells is long and tedious and is not suited for the commercial production of such filters. Accordingly numerous approaches to simplify and speed the plugging operation have been developed. One such approach, disclosed in U.S. Pat. No. 5,021,204, involves the use of a rigid solid masking plate having a number of openings or bores extending therethrough for directing suitable plugging materials into the ends of the channels to be plugged.
Many alternative masking approaches employing flexible polymeric masks or masking strips of impermeable and/or re-useable materials have also been proposed. U.S. Pat. No. 4,411,856 provides some examples. Further, a wide variety of compositions for the plugging materials to be used in these processes is known, including the foamed cement plugging materials disclosed in U.S. Pat. No. 4,297,140.
A common disadvantage of most of the plugging processes and materials developed to date is that they cannot successfully be applied to ceramic honeycombs that have not been fired or otherwise processed to develop high wall strength. Thus many of the known plugging processes and materials are suitable only for application to pre-fired honeycombs, and further require a second firing treatment following the introduction of the plugging materials to cure and bond the plugs firmly into the honeycomb structure. These extra firing steps add significant cost to the filter manufacturing process.
Another disadvantage of many of the known plugging formulations is that they require the application of significant extraneous heat or other energy in order to achieve sufficient adherence and strength to withstand further handling in the course of manufacture. So-called “cold set” plugging cements that would form a durable mechanical seal without additional processing have not been widely developed or used. Further, some cold set cements that have been proposed for use employ ceramic fiber additives for the purpose of plug reinforcement, and such additives are generally to be avoided for environmental reasons.
Finally, any sealing system to be used for the manifolding of ceramic honeycomb filter bodies must exhibit chemical and physical stability sufficient to withstand extended use under the conditions encountered by the filter bodies in use. These conditions include high temperatures and chemical reactive environments.